Many attempts have been made to prepare hybrid polymer dispersions, in particular alkyd-vinyl hybrid polymer dispersions, aiming to achieve an ideal balance of the best properties of each of the polymers combined in the hybrid polymer. Alkyd emulsions and vinyl polymer dispersions, made by means of emulsion polymerization, are the two most common waterborne technologies used in the coating industry. Coatings based on alkyd emulsions exhibit advantages over those based on vinyl polymer dispersions in terms of gloss, brush-ability, open time, flow and leveling, hiding per coat, mar resistance, sand-ability, and adhesion. Advantages of vinyl polymer dispersions over alkyd emulsions include quick drying, durability, non-yellowing, non-embrittling, and non-chalking (pigment release by degradation of the binder).
In addition to the advantages described above for alkyds, coatings based on alkyd-modified hybrid dispersions have improved film forming properties due to the presence of alkyd moieties, which typically have a low glass transition temperature (Tg). However, alkyd resins and vinyl polymers are incompatible, and hence prone to suffer phase segregation that results in haziness and lower gloss. Compatibility can be improved by enhancing grafting of the vinyl polymer onto the alkyd resin but this causes the loss of the unsaturated groups of the alkyd, which results in the deterioration of the auto-oxidation properties of the alkyd. The presence of the phase-separated alkyd also results in critical problems, such as soft films (causing dirt pick-up and poor block resistance) and yellowing. High residual monomer content is a threat to the environment and health and is strictly regulated. Furthermore, during the addition polymerisation, unsaturated groups on the alkyd are lost, resulting in the deterioration of the auto-oxidation properties of the alkyd. The known hybrids also have an undesirably low open time, defined as the period of time during which corrections can be made in the wet paint film without leaving brush marks.
From U.S. Pat. Nos. 3,919,145, 4,108,811 and WO9726303 it is known that blending oxidatively drying polymers with acrylic polymer dispersions reduce yellowing and in some cases cost. However the performance of the resulting coatings, in particular the mechanical properties and the resistance to chemicals is greatly reduced. U.S. Pat. No. 6,001,922 discloses a blend of polyester/acrylic hybrid latexes with an alkyd.
T. Nabuurs and A. German, Progress in Organic Coatings, Vol. 27, pages 163-172 (1996) discuss that a mixture of alkyd and vinyl polymers can exist as a blend of a vinyl polymer dispersion and an alkyd dispersion, however although the dispersions are blended, the alkyd and vinyl polymers each exist in distinct particles or phases causing application problems like a lower gloss. (see also “Alkyd-acrylic composite emulsions: polymerization and morphology”, Tijs Nabuurs, doctoral thesis, Technische Universiteit Eindhoven, 1997, ISBN 90-386-0978-7).
A more effective way to form an intimate dispersion could be expected to be by forming the acrylic polymer in-situ, namely by polymerizing the acrylic monomers in the presence of the alkyd dispersion. U.S. Pat. No. 3,847,851 discloses the preparation of solvent based dispersions prepared by polymerizing monomers in the presence of alkyd resins, but large amounts of volatile organic compounds are used. U.S. Pat. No. 4,413,073 discloses the preparation of dispersions of film forming polymer produced by polymerization of monomers in the presence of preformed polymers. Wang et al., J. Appl. Polym. Sci, vol 60 pages 2069-2076 (1996) discuss the emulsion and mini-emulsion copolymerization of acrylic monomers in the presence of alkyds. However the through-dry time (oxidation of the film) of such coatings is longer due to the decreased degree of unsaturation in the alkyd resulting from the copolymerization with the vinyl monomers.
WO 98/13404 discloses the preparation of a polyurethane-alkyd resin/acrylate aqueous dispersion, which is prepared by first forming the polyurethane-alkyd resin in methyl ethyl ketone. After neutralization of the polyurethane-alkyd resin, it is dispersed in water, and then the methyl ethyl ketone removed. After that, at least one olefinically unsaturated monomer substantially having no reactive functional groups other than the double bonds is polymerized in said dispersion. These dispersions have the disadvantage that compatibilization between the polyurethane-alkyd resin and the acrylate polymer is limited, which would result in phase separation. Moreover, the azeotropic distillation of relatively large amounts of solvents present safety, environmental and economic problem. Also they use relatively expensive polyurethanes.
WO0228977 describes an aqueous dispersion comprising an alkyd, a vinylpolymer I, a vinylpolymer II and a liquid medium; wherein said vinyl polymer I is pre-formed prior to incorporation into the aqueous dispersion and wherein said vinylpolymer II is prepared in the presence of the alkyd and the vinylpolymer I. These aqueous dispersions are claimed not to suffer from impaired auto-oxidation properties of the alkyd. This process will not result in the formation of a true hybrid polymer with the disadvantages as described above.
U.S. Pat. No. 6,369,135 describes a hybrid graft copolymer made by dissolving an alkyd resin in various vinyl monomers (methyl methacrylate, styrene, etc.). The monomer/alkyd solution is then mini-emulsion polymerized to form a latex consisting of submicron particles of polymer with the alkyd grafted onto the polymer backbone. The mini-emulsion process is described in the following publications: J. M. ASUA. “Mini-emulsion Polymerization”. Prog. Polym. Sci. 27,1283 (2002) and I. Capek and C. S. Chern, “Radical Polymerization in Direct Mini-Emulsion Systems”, Advances in Polymer Science, Vol. 155 (2001). The disadvantage of this process is the high residual monomer content and no compatibilization unless a high amount of double bonds of the alkyd are consumed (which would result in a substantial loss of the autoxidation properties of the alkyd).
WO9502019 discloses hybrid emulsions comprising an air-drying alkyd resin, combined with an acrylic resin, dispersed in water. The hybrid emulsions are prepared by first mixing an alkyd emulsion with an acrylic monomer to yield a pre-emulsion. The pre-emulsion is then further emulsified in water in high-pressure emulsifying equipment. The resulting pre-emulsion of the alkyd/acrylic mixture is then heated to 80° C., and an initiator added for a batch emulsion polymerization process. The process of this patent reaches a low limiting conversion resulting in an aqueous dispersion having a high level of residual monomers that is not fit for use in coatings.
U.S. Pat. No. 3,620,989 describes a process for preparing an aqueous dispersion of multi-polymer particles wherein the individual particles comprise both alkyd resin and addition polymer. The alkyd resin having unsaturation in the polymeric backbone derived from the use of an unsaturated diacid or anhydride and the vinyl monomer are emulsified and copolymerized. This polymerization has, however, a relatively poor conversion rate. The examples show a percentage yield of polymer of 85% maximum which means that the compositions contain a high level of free monomer. Such a high level is unacceptable from both toxicological and customer acceptance points of view.
EP0608020 discloses the preparation of aqueous dispersions of core/shell polymers having an acrylic-grafted alkyd resin as the core and an addition polymer as the shell. An alkyd resin is prepared first by conventional means. Next, the alkyd resin is then dissolved in a water-miscible organic solvent and reacted with vinyl monomers, containing at least one of them a carboxyl functionality. The solution of the acrylic grafted alkyd resin is neutralized, and dispersed in water. The dispersion thus obtained may be used as seed in an emulsion polymerization process in order to form an acrylic shell around the alkyd-acrylic graft copolymer. Only limited grafting between the alkyd and the vinyl polymer can be achieved without consuming too many fatty-acid double bonds of the alkyd (which would result in a substantial loss of the autoxidation properties of the alkyd).
U.S. Pat. No. 6,627,700 discloses the preparation of an aqueous dispersion of an acrylate modified alkyd resin for which at least one alkyd containing allyloxy group is dispersed in water containing at least one water-miscible diol such as propylene glycol. The diol is used as a solvent to reduce the viscosity of the alkyd. The water-miscible diols in the patent are organic solvents that contribute to the VOC of the aqueous dispersion. Furthermore the acid value of the alkyd is at least 35 mg KOH/g. Alkyd resins having an acid value higher than 5 are known to have poor hydrolytic stability.
EP0295403B1 discloses water-soluble alkyd resins, converted into the water-soluble form by neutralization of their methacrylic-carboxyl groups, derived from graft copolymers of an unsaturated fatty acid with mixtures of methacrylic acid and acryloyl-methacryloyl-vinyl monomers. Both the graftcopolymer and the acrylic-alkyd copolymer are made by a process utilizing organic solvents. The obtained aqueous acrylic-alkyd emulsion is not free of organic solvents.
WO99/07799 discloses the preparation of an acrylic modified aqueous alkyd dispersion, which is prepared by the polymerization of at least one latent oxidatively-functional acrylic monomer in the presence of a water-reducible alkyd resin. Acrylic monomers having a latent oxidatively-functional group are monomers containing one reactive double bond (e.g., methacrylate) and one non-reactive double bond (e.g., allylic), which after polymerization possess sufficient unreacted double bonds to increase or amplify the degree of crosslinking normally found in alkyd resins. The disadvantages of this approach are: i) the waterborne alkyd provides undesirable water sensitivity to the coating; ii) the contact between the acrylic polymer and the waterborne alkyd is limited because these polymers are incompatible and hence the interaction between the latent oxidatively-functional groups and the unsaturations of the alkyd resin during film formation is reduced. Furthermore the examples given contain an organic solvent.
The object of the invention therefore is to provide a process for the manufacture of a hybrid of an auto-oxidatively crosslinkable polymer and an addition polymer that does not have at least one of the abovementioned disadvantages, in particular a process and resulting product that combines the good properties of both the AOC polymer and the addition polymer in a single composition but with a reduced phase segregation tendency both of the composition itself (storage stability) as well as on drying, resulting for example in better drying properties, low (no) volatile organic compounds and coatings having good chemical resistance, mechanical properties and optical appearance (high gloss) and good film forming properties.